Spray-On Flat Lens Works in the UV

Photonics.comMay 2013
GAITHERSBURG, Md., May 24, 2013 — A metamaterial-based flat lens that's easy to fabricate and that can bend and focus UV light could improve photolithography, nanoscale manipulation and manufacturing, and even high-resolution 3-D imaging, say scientists at the National Institute of Standards and Technology (NIST).

The new lens is formed by making a simple sandwich of alternating nanometer-thick layers of silver and titanium dioxide. The resulting flat slab is a metamaterial — a man-made substance with a negative index of refraction: Light entering or exiting the material bends in a direction opposite what would occur in almost all other materials in almost all other materials.

In addition to having a negative refractive index and the ability to bend and focus ultraviolet light, the lens is also uniquely flat. Nearly all lenses today — whether in an eye, a camera or a microscope — are curved.

"Curved lenses always have a limited aperture," said team member Kenneth Chau of the University of British Columbia. "With a flat lens, suddenly you can make lenses with an arbitrary aperture size — perhaps as big as a football field."

The NIST team created a UV metamaterial formed of alternating nanolayers of silver (green) and titanium dioxide (blue). When illuminated with UV light (purple), a sample object of any shape placed on the flat slab of metamaterial is projected as a 3-D image in free space on the other side of the slab. Here, a ring-shaped opening in an opaque sheet on the left of the slab is replicated in light on the right. Courtesy of Lezec/NIST.
A Russian physicist, Victor Veselago, first described the idea of a flat lens created from a negative-index material in a 1967 paper, but it took more than 30 years for such a material to be created.

"The challenge is that there are no naturally occurring materials to make that type of flat lens," Chau said. "Through trial and error, and years of research, we have come up with a fairly simple recipe for a spray-on material that can act as that flat lens."

For the past decade, scientists have made metamaterials that work at microwave, infrared and visible wavelengths by fabricating repeating metallic patterns on flat substrates. However, the smaller the wavelength scientists want to manipulate, the smaller these features need to be, which makes fabricating the structures an increasingly difficult task. Until now, making metamaterials that work in the UV has been impossible because it requires making structures with features as small as 10 nm.

Because of their inherent limitations, metamaterials of this type designed for infrared and visible wavelengths have, so far, been shown to impart a negative index of refraction to light that is traveling only in a certain direction, making them hard to use for imaging and other applications that rely on refracted light.

To overcome this, the NIST team took inspiration from a theoretical metamaterial design recently proposed by a group at the FOM Institute for Atomic and Molecular Physics in Holland. They adapted the design to work in the UV — a frequency range of particular technological interest.

Because their lens doesn't rely on nanoscale patterns, it is inherently easy to fabricate, the authors said.

Kenneth Chau, University of British Columbia, is excited about the newly published research that explains how he and his colleagues at NIST developed a metamaterial that can be sprayed onto a surface, where it acts as a lens. Courtesy of University of British Columbia.
"Our lens will offer other researchers greater flexibility for manipulating UV light at small-length scales," said team member Henri Lezec. "With its high photon energies, UV light has a myriad of applications, including photochemistry, fluorescence microscopy and semiconductor manufacturing. That, and the fact that our lens is so easy to make, should encourage other researchers to explore its possibilities."

The metamaterial flat lens achieves its refractive action over a distance of about two wavelengths of UV light — a focal length challenging to achieve with conventional refractive optics such as glass lenses. Also, transmission through the metamaterial can be turned on and off using higher-frequency light as a switch, allowing the flat lens also to act as a shutter with no moving parts.

The new work was performed in collaboration with researchers at the Maryland NanoCenter of the University of Maryland, Syracuse University and the University of British Columbia.

"This is the closest validation we have of the original flat lens theory," Chau said. "The recipe, now that we've got it working, is simple and cost-effective. Our next step is to extrapolate this technique further, explore the effect to the fullest, and advance it as far as we can take it."

A material engineered from artificial matter not found in nature. The artificial makeup and design of metamaterials give them intrinsic properties not common to conventional materials that are exploited as light waves and sound waves interact with them. One of the most active areas of research involving metamaterials currently explores materials with a negative refractive index. In optics, these negative refractive index materials show promise in the fabrication of lenses that can achieve...